I got the answer from a certifier:"The gold colour turns green when the stamp has been in contact with plastic."
It seems a good explanation but...
There are two kind of stockbooks with plastic or pergamin strips. I have both stockbook types, but stamp was stored in a book with pergamin strips, so the change had taken place before I got the stamp about 15–20 years ago.
Usually strips don't cover the whole stamp (depending on the stamp size), so if the stamp was kept in a stockbook with plastic strips, only the lower half would have been in contact with plastic. The are, however, also loose pages which cover almost the whole stamp.
Even if the stamp was kept in a loose page like above, what would explain the offset on the backside? Would the contact with plastic start a chemical prosess which makes the changed ink penetrate through the paper?
Leuchtturm claim that their mounts, strips, stockbooks etc. won't harm stamps.
@mike2006 No other colours, only green.